Michelle:
Following up on the great leads from Col, I did find a Rathangan townland in Duncormick civil parish in Co. Wexford.,I obtained the 1853 Griffiths Valuation for that parish. There is a Mary Larkin listed in Rathangan.
http://www.failteromhat.com/griffiths/wexford/duncormick.htm
This relates to the Ancestry tree that Col located with an 1842 Catherine with parents named Thomas and Mary. If Thomas had died before 1853 then Mary would have been the lease holder of the land. ( I recognize that you are looking for Lawrence and Margaret as parents).
Roots Ireland has only 6 RC parishes online and does not have Rathangan parish (which is the Catholic parish corresponding to Duncormick). A research book I have says that Rathangan records go back to 1803. You may want to contact the church to see if they can help.
http://www.ferns.ie/rathangan/
Roger McDonnell